1. Field of the Invention
This invention pertains to a method of recovering liquid hydrocarbons from a molecular sieve sorbent bed in a hydrocarbon treating system. This invention is particularly useful in recovering "sponge" or "sponged" propane that has been retained by a bed of zeolitic molecular sieves in a treatment system designed to remove sulfur compounds (e.g., carbonyl sulfide) from liquid propane.
2. Description of the Prior Art
Various methods have been used to remove sulfur compounds (e.g., carbonyl sulfide (COS), carbon disulfide (CS.sub.2), hydrogen sulfide (H.sub.2 S), and the like) from hydrocarbon streams. Such methods have been referred to as "sweetening" or "desulfurization" processes.
The reasons for removing sulfur compounds from hydrocarbon streams are almost as diverse as the sweetening methods used. Two of the primary reasons are (a) to protect the environment by eliminating or reducing the amount of noxious/toxic hydrogen sulfide and/or sulfur dioxide formed during combustion, and (b) to protect hydrocarbon process equipment that comes in contact with such hydrocarbon streams. Sulfur compounds, particularly H.sub.2 S, are known to corrode base metals (metal parts) and to foul catalysts used in refineries.
The specification limits upon the maximum amount of sulfur compounds that may be present in a particular hydrocarbon product will vary depending upon the product and its conventional use. Increasingly stringent limits have been placed on liquid hydrocarbons conventionally used as fuels or fuel additives. E.g., gasoline and diesel fuels in the transportation area, and liquid propane and butane fuels in combustion heating.
One method that has been used to sweeten liquid hydrocarbons involves contacting the liquid hydrocarbon stream with one or more sorbent beds loaded with materials that sorb the sulfur compound(s) as the hydrocarbon stream passes through the bed. Various sorbents have been used, but molecular sieves have been particularly useful in this application. The molecular sieves are typically zeolitic materials, both natural and synthetic, comprising ordered porous crystalline aluminosilicates having a definite crystalline structure, as determined by X-ray diffraction within which there are a large number of small cavities, which are interconnected by a series of still smaller channels or pores having a uniform pore size or pore size distribution in a rigid three-dimensional matrix of SiO.sub.4 and AlO.sub.4. Such molecular sieves and the methods of making and using them in sweetening hydrocarbon streams are well known, as illustrated by the following nonexhaustive list of patents:
______________________________________ Assigned to Union Carbide Others ______________________________________ USP USP USP USP 3,069,362 4,645,516 3,051,646 4,313,821 3,069,363 4,683,217 3,102,855 4,404,118 3,211,644 4,684,511 3,188,293 4,540,842 3,620,969 4,684,617 3,218,250 4,703,025 3,654,144 4,686,029 3,282,831 3,660,967 4,686,092 3,450,629 4,098,684 4,686,093 3,490,865 4,738,837 ______________________________________
The disclosures of these patents are incorporated herein by reference to the extent that any such patent, or combination of patents, describes a molecular sieve, or a method of making a molecular sieve, that can be used to remove sulfur compounds from a liquid hydrocarbon stream. As these patents indicate, the zeolitic materials can be manufactured with a uniform pore size and with a pore size chosen to selectively adsorb molecules of certain dimensions (e.g., COS) while rejecting molecules of larger dimensions (e.g., propane)--hence, the term "molecular sieve."
Under conditions of use in sweetening operations, a sour stream of liquid hydrocarbons is contacted with the molecular sieves under conditions sufficient to remove appreciable quantities of the sulfur compound(s) from the hydrocarbon stream. Typically, a pressurized hydrocarbon stream is passed through one or more "packed columns" or "beds" of molecular sieves at a flow rate and other process conditions that provides adequate contact time for the sulfur compounds to be removed. As the molecular sieves reach the limit of their capacity to sorb the sulfur compounds (i.e., the molecular sieves become "loaded") and prior to "breakthrough" when the treated hydrocarbon product passing through the molecular sieves fails to meet specifications with respect to the amount of sulfur compounds in the product, the flow of the hydrocarbon is stopped until the bed of molecular sieves can be regenerated, or more typically, the flow is diverted to another bed of molecular sieves while the first bed is regenerated.
The bed of molecular sieves that is loaded with sorbed sulfur species can be regenerated by draining liquid hydrocarbon from the bed and then heating the bed of molecular sieves under conditions sufficient to volatilize and desorb the sulfur species and thereby reactivate the molecular sieves to further sorption. The vapors from the heated bed are typically flared. This technique for regeneration is well known and widely used in the industry. Unfortunately, this technique results in a substantial loss of hydrocarbon product, i.e., material that does not drain from the bed and is lost to the flares when the bed is heated. The loss of product retained by the molecular sieves (sometimes referred to as "sponge" or "sponged" hydrocarbon) is exacerbated when the sulfur content in the hydrocarbon feed stream is high and/or when the specification on the treated product is stringent. In such instances, the bed must be regenerated more frequently.
A method has now been discovered that will reduce this loss of hydrocarbon and thereby enhance the economics of the treatment process.